This invention relates to the field of framing structures for digital subscriber lines, and more particularly to frames having a payload portion supporting imbedded synchronous and asynchronous traffic for communication over a digital subscriber line.
Communication systems are used to transmit and receive signals over a communication channel. One common form of communication system is a telephone system for transmitting and receiving voice and data signals. In a digital communication system signals representing digital data are sent over a transmission medium. A digital subscriber loop/line (DSL) is a digital telecommunications system operating over existing copper telephone lines (unshielded twisted pair-UTP), as opposed to optical fiber.
Digital signals typically have greater capacity of information and less noise problems than analog signals. The data transmitted over a DSL is typically organized according to a specific format so that two communicating parties are able to recognize and retrieve the data using the same format. To accomplish this, data is organized into frames. Frames are blocks of serial information that are transmitted consecutively, where each frame generally includes the same amount of data. The data in a frame is typically surrounded by framing information for synchronization and error checking.
Typical DSL technologies to support the requirement of independent and simultaneous voice and data transmission over the same phone line include: asymmetrical digital subscriber line (ADSL) and rate adaptive digital subscriber line (RADSL) technologies. ADSL that allows for the transmission of about 6 Mbps over existing UTP (1.544 to 6.144 Mbps downstream (i.e. central office to customer) and 16 to 640 kbps upstream (i.e. customer to central office).
Voice as an analog signal is handled at each end of the connection through a splitter which ensures that it is unaffected by the presence or absence of digital data. In this way, the voice signal and the digital data signal are transmitted simultaneously over the twisted pair while occupying different frequency bands and not interfering with each other. Video and data inputs (asynchronous traffic) are time division multiplexed into the digital ADSL signal which originates in an ADSL transmission unit-central office (ATU-C). The signal then travels the local loop, terminating in an ADSL Transmission Units-Remote (ATU-R) located on customer premises.
There is an increasing demand for more phone lines to be located at the customer premises to support multiple telephones and/or computers or facsimile machines. This creates a need to carry the voice band traffic in a digital form and embed it in the data stream. This allows a reduction in the number of twisted pairs entering the dwelling, but imposes new requirements on the data path. Digitized voice requires minimal transport delay and synchronous operation in that each voice sample must be received at a precise time interval.
The rate adaptive digital subscriber line (RADSL) technology is an advance over other DSL implementations because its transmission speed is rate-adaptive based on the length and signal quality of an existing telephone line. Products based on RADSL have the option of selecting the highest practical operating speed automatically or as specified by a service provider. However, this rate adaptation can impact the quality of the digital voice channels that require a precise and constant data rate. The variation of the loop signal data rate must be totally absorbed by the data channel traffic. In addition, there are situations where the data rate allocated to the voice traffic must be changed, such as when a communication terminates or if a new one begins.
Consequently, there is a need to provide a DSL framing structure supporting imbedded rate adaptive synchronous and asynchronous traffic that can be automatically adjusted to (a) provide more bandwidth for synchronous traffic when the synchronous load is increased; (b) reduce the bandwidth for synchronous traffic when the demand for synchronous use is reduced; and (c) adjust the bandwidth for asynchronous traffic when the line quality increases or decreases.
An object of the present invention is to provide a DSL framing structure capable of supporting rate adaptive allocation of bandwidth to synchronous traffic and to asynchronous traffic over the same frame structure.
Another object of the present invention is to provide a method for adding or removing synchronous traffic dynamically to a DSL frame structure as opposed to maintaining a fixed synchronous/asynchronous traffic mix.
Another object of the present invention is to provide a DSL framing structure that provides a minimal delay path for synchronous traffic.
In accordance with an aspect of the present invention there is provided a frame having a payload portion supporting imbedded synchronous and asynchronous traffic for communication over a digital subscriber line having a prescribed data rate, comprising: (a) a first payload group for the synchronous traffic in the payload portion of the frame, said first payload group having a prescribed size;(b) a second payload group for the asynchronous traffic in the payload portion of the frame, said second payload group having a prescribed size; and (c) means for modifying the prescribed size of the first payload group and the prescribed size of the second payload group.
In accordance with another aspect of the present invention there is provided in a communication system transmitting a prescribed data rate over a digital subscriber line for simultaneously accommodating synchronous traffic from a synchronous device and asynchronous traffic from a asynchronous device, a frame having a payload portion comprising: (a) a plurality of first payload groups of width U bytes for the synchronous traffic in the payload portion of the frame; (b) a plurality of second payload groups of width C bytes for the asynchronous traffic in the payload portion of the frame; and (c) an algorithm for defining and modifying U and C for adapting to variations in demand for synchronous traffic over the digital subscriber line.
In accordance with another aspect of the present invention there is provided in a communication system having a digital subscriber line having a prescribed data rate and a signal-to-noise ratio (SNR), a method for simultaneously accommodating synchronous traffic from a synchronous device and asynchronous traffic from a asynchronous device, said method comprising: (a) alternating a plurality of first payload groups of width U bytes for synchronous traffic with a plurality of second payload groups of width C bytes for the asynchronous traffic in the payload portion of the frame; and (b) modifying U and C for adapting to variations in demand for synchronous traffic over the digital subscriber line.